make PHPCR ODM 5.6 compatible

fixing the bug with creating new class instances

fixes https://github.com/doctrine/phpcr-odm/issues/534

however we need to wait for Doctrine to release a stable version but imho we should already merge this.
The current point of instantiator is replacing a bad hack with a different bad hack (but maintained in its own repo).

I see little use-case for swapping out the instantiator here. If it is the case, then a setter would probably be better. 

Additionally, since a `ClassMetadata` instance is usually un-serialized, the instantiator should either be re-built on `__wakeup` (therefore no DI going on) or via `wakeupReflection` (the `ClassMetadataFactory` has to re-inject it after unserialization)